What this PR does / why we need it:

On-Demand Feature Views (ODFVs) with write_to_online_store=True were not persisting transformed values during materialization.

The root causes were:

• Missing ODFV materialization logic in FeatureStore.materialize() and materialize_incremental() methods.
• Incorrect type detection in utils._convert_arrow_to_proto() causing ODFVs to be treated as regular FeatureViews.

Added integration tests covering:

• Single-source ODFV materialization
• Multi-source ODFV materialization
• Incremental materialization with ODFVs
• Mixed on-write/on-read ODFV scenarios (reproduces original issue)
• Multiple write methods (materialize, write_to_online_store, etc.)

Which issue(s) this PR fixes:

Fixes #5430